stat-util: add macros for checking whether major and minor values are in range

As it turns out glibc and the Linux kernel have different ideas about
the size of dev_t and how many bits exist for the major and the minor.
When validating major/minor numbers we should check against the kernel's
actual sizes, hence add macros for this.

src/basic/stat-util.h

@@ -62,3 +62,22 @@ int fd_verify_regular(int fd);
 
 int stat_verify_directory(const struct stat *st);
 int fd_verify_directory(int fd);
+
+/* glibc and the Linux kernel have different ideas about the major/minor size. These calls will check whether the
+ * specified major is valid by the Linux kernel's standards, not by glibc's. Linux has 20bits of minor, and 12 bits of
+ * major space. See MINORBITS in linux/kdev_t.h in the kernel sources. (If you wonder why we define _y here, instead of
+ * comparing directly >= 0: it's to trick out -Wtype-limits, which would otherwise complain if the type is unsigned, as
+ * such a test would be pointless in such a case.) */
+
+#define DEVICE_MAJOR_VALID(x)                                           \
+        ({                                                              \
+                typeof(x) _x = (x), _y = 0;                             \
+                _x >= _y && _x < (UINT32_C(1) << 12);                   \
+                                                                        \
+        })
+
+#define DEVICE_MINOR_VALID(x)                                           \
+        ({                                                              \
+                typeof(x) _x = (x), _y = 0;                             \
+                _x >= _y && _x < (UINT32_C(1) << 20);                   \
+        })